[llvm-dev] [RFC] llvm-exegesis: Automatic Measurement of Instruction Latency/Uops
Clement Courbet via llvm-dev
llvm-dev at lists.llvm.org
Thu Mar 15 08:49:18 PDT 2018
On Thu, Mar 15, 2018 at 4:41 PM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On 03/15/2018 10:04 AM, Guillaume Chatelet via llvm-dev wrote:
>
> [You can find an easier to read and more complete version of this RFC here
> <https://docs.google.com/document/d/1QidaJMJUyQdRrFKD66vE1_N55whe0coQ3h1GpFzz27M/edit?ts=5aaa84ee#>
> .]
>
> Knowing instruction scheduling properties (latency, uops) is the basis for
> all scheduling work done by LLVM.
>
> Unfortunately, vendors usually release only partial (and sometimes
> incorrect) information. Updating the information is painful and requires
> careful guesswork and analysis. As a result, scheduling information is
> incomplete for most X86 models (this bug
> <https://bugs.llvm.org/show_bug.cgi?id=32325> tracks some of these
> issues). The goal of the tool presented here is to automatically
> (in)validate the TableDef scheduling models. In the long run we envision
> automatic generation of the models.
>
> At Google, we have developed a tool that, given an instruction mnemonic,
> uses the data in `MCInstrInfo` to generate a code snippet that makes
> execution as serial (resp. as parallel) as possible so that we can measure
> the latency (resp. uop decomposition) of the instruction. The code snippet
> is jitted and executed on the host subtarget. The time taken (resp.
> resource usage) is measured using hardware performance counters. More
> details can be found in the ‘implementation’ section of the RFC.
>
> For people familiar with the work of Agner Fog, this is essentially an
> automation of the process of building the code snippets using instruction
> descriptions from LLVM.
> Results
>
> -
>
> Solving this bug <https://bugs.llvm.org/show_bug.cgi?id=36084>
> (sandybridge):
>
> > llvm-exegesis -opcode-name IMUL16rri8 -benchmark-mode latency
>
> ---
>
> asm_template:
>
> name: latency IMUL16rri8
>
> cpu_name: sandybridge
>
> llvm_triple: x86_64-grtev4-linux-gnu
>
> num_repetitions: 10000
>
> measurements:
>
> - { key: latency, value: 4.0115, debug_string: '' }
>
> error: ''
>
> ...
>
> > llvm-exegesis -opcode-name IMUL16rri8 -benchmark-mode uops
>
> ---
>
> asm_template:
>
> name: uops IMUL16rri8
>
> cpu_name: sandybridge
>
> llvm_triple: x86_64-grtev4-linux-gnu
>
> num_repetitions: 10000
>
> measurements:
>
> - { key: '2', value: 0.5232, debug_string: SBPort0 }
>
> - { key: '3', value: 1.0039, debug_string: SBPort1 }
>
> - { key: '4', value: 0.0024, debug_string: SBPort4 }
>
> - { key: '5', value: 0.3693, debug_string: SBPort5 }
>
> error: ''
>
> ...
>
> Running both these commands took ~.2 seconds including printing.
>
>
>
> -
>
> List of measured latencies
> <https://docs.google.com/spreadsheets/d/11_vFQRpiPHQ3zLcx8cVYYCqR5N5PCa4IvMyKHwF7Op4/edit?usp=sharing>
> for sandybridge, haswell and skylake processors including diffs with LLVM
> latencies. Excerpt:
>
>
>
> sandybridge
>
> haswell
>
> skylake
>
> mnemonic
>
> llvm-exegesis
>
> TD file
>
> llvm-exegesis
>
> TD file
>
> llvm-exegesis
>
> TD file
>
> SHR32r1
>
> 1.01
>
> 1.00
>
> 1.00
>
> 1.00
>
> 1.01
>
> 1.00
>
> IMUL16rri
>
> 4.02
>
> 3.00
>
> 4.01
>
> 3.00
>
> 4.01
>
> 3.00
>
>
> -
>
> Some instructions have different implementations depending on which
> registers are assigned. This is well known for cases like `xor eax,
> eax` and `xor eax, ebx`, which emits no uops in the first case (this
> happens during register renaming, see Agner Fog’s “Register Allocation and
> Renaming”, in microarchitecture.pdf
> <http://www.agner.org/optimize/microarchitecture.pdf>). But we found
> out that this can go further. For example, SHLD64rri8 takes one cycle
> and runs on P06 in the `shld rax, rax, 0x1` case, but takes 3 cycles
> and runs on P1 in the `shld rbx, rax, 0x1` case. To the best of our
> knowledge, this has not yet been described.
>
>
> This is great!
>
> Future Work
>
> -
>
> [easy] Fix Intel Scheduling Models.
> -
>
> [easy] Extend to memory operands.
> -
>
> [easy] Make the tool work reliably for x87 instructions.
> -
>
> [medium] A tool that automatically create patches to TD files.
> -
>
> [medium] Measure the effect of immediate/register values: Some
> instructions have performance characteristics that depends on the values it
> operates on. We should explore the value space (0, 1, ~1, 2^{8,16,32,64},
> inf, nan, denorm...).
> -
>
> [medium] Measure the effect of changing registers on instruction
> implementation (see results section
> <https://docs.google.com/document/d/1QidaJMJUyQdRrFKD66vE1_N55whe0coQ3h1GpFzz27M/edit?ts=5aaa84ee#bookmark=kix.q6a0imw9qn1n>
> above). Model this in LLVM TD schema.
> -
>
> [hard] Make the tool work for instruction that have side effects (e.g.
> PUSH/POP, JMP, ...). This might involve extending the TD schema with
> information on how to setup measurements for specific instructions.
> -
>
> [??] Make the tool work for other CPUs. This mainly depends on the
> presence of performance counters.
>
> Open Questions We depend on libpfm
> <http://perfmon2.sourceforge.net/docs_v4.html>. How do we handle the
> dependency ?
>
>
> Are there options that you have in mind? It's an external MIT-licensed
> dependency. Wouldn't CMake just detect it when it's available?
>
That's what we've done for now (see code here
<https://reviews.llvm.org/differential/changeset/?ref=1002469&whitespace=ignore-most>).
We're not sure what the policy is wrt external deps. Right now if the tool
is enabled and libpfm is not on the system, we die with an error message.
The other options would be to disable the tool in that case (I'm not sure
how to do that). Opinions ?
>
> -Hal
>
> --
> Guillaume Chatelet (gchatelet at google.com), Clement Courbet (
> courbet at google.com) for the Google Compiler Research Team
>
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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